The present invention is particularly applicable for providing a colored, protective barrier on 304 or 316 stainless steel used for roofing material or other architectural material and it will be described with particular reference thereto; however, the invention has much broader applications and can be used for various stainless steel and various articles in strip form or otherwise. "Stainless steel" in the application means a large variety of alloy metals containing chromium and iron. The alloy may also contain nickel, carbon, molybdenum, silicon, manganese, titanium, boron, copper, aluminum, nitrogen and other various elements and compounds. For many years, roofing systems made of metal in various sheet gauge thicknesses have been used. Metals such as carbon steel and stainless steel are the most popular types of metal roofing systems. Carbon steel metal roofing systems are commonly treated with a corrosion-resistant coating to prevent rapid oxidation of the iron. One type of corrosion-resistant coating for carbon steel is a tin metal coating used in the food industry. Tin coating of carbon steel is normally carried out by a continuous, high-speed electrolysis process. In an electrolysis process, an electrical current is used to reduce alkaline or acidic electrolytes of tin to plate the tin on the carbon steel. The thickness of the tin coating ranges between 3.8.times.10-4 to 20.7.times.10-4 mm (1.5.times.10-5-8.15.times.10-5 in.). The equipment and materials used to electroplate carbon steel are very expensive and relatively complex to use; however, only a thin layer of tin is used so the cost of the expensive tin maintained is quite low. A less used process of coating carbon steel is by a hot dipping process. This process is normally not used because of the resulting minute areas of discontinuity in the tin coating. Consequently, the material is less satisfactory for food containers. In addition, hot dipped tin forms a thicker coating which is prone to flaking. Because tin is a material that is resistant to corrosion, materials highly susceptible to corrosion such as carbon steel can be coated with tin to produce highly corrosive-resistant products.
Many metallic alloys have been developed which have increased resistance to corrosion, such as stainless steel. Stainless steel is an alloy of iron and chromium and may include nickel and molybdenum and small amounts of other elements. The chromium within the stainless steel alloy is one of the primary components which inhibits corrosion. The chromium forms chromium oxide and tightly bonds to the surface of the stainless steel thus preventing oxygen from penetrating into the stainless steel to form corrosive ferrous oxides. Carbon steel has little if any chromium content, thus the iron readily oxidizes with the surrounding oxygen to form ferrous oxides commonly known as corrosion. Although stainless steel corrodes at a significantly slower rate than standard carbon steel, the stainless steel will eventually corrode and will corrode at a significantly faster rate than carbon steel coated with tin plate. Stainless steel is highly suceptable to corrosion in seawater where the salts really attack and corrode the stainless steel because of the chlorine in the environment.
Coating stainless steel with tin alloys by a hot-dipped process has been more successful. One of the most popular tin alloy coatings for carbon steel and stainless steel is a tin-lead alloy commonly known as terne. The composition of the terne alloy is generally about 80 weight percent lead and about 20 weight percent tin. The lead in the terne alloy readily bonds to both carbon steel and stainless steel to form a strong and durable lead-tin alloy coating. Although terne coated sheet metals have excellent corrosive-resistant properties and have been used in a wide variety of building applications such as roofing, terne coated materials have recently raised environmental concerns due to the lead content of the terne alloy. Even though the lead in the terne alloy is stabilized, there is some concern, albeit unfounded, about leaching of the lead from the terne alloy.
In U.S. application Ser. No. 000,101, a process for successfully coating stainless steel materials with tin containing little, if any, lead is disclosed. The tin coatings achievable are significantly thicker than thickness obtained by the electroplating process. Although the tin coating is more resistant to corrosion than stainless steel in a marine or saline environment, the tin still corrodes at an accelerated rate in such environments thus reducing the demand of tin coated products in such environments. Buildings located in costal regions throughout the world are subjected to a saline environment. Such regions must deal with above average rainfall and the saline environment resulting from the nearby seawater. The saline environment readily attacks metals such as iron and stainless steel thereby accelerating the corrosion rates. Structures that are located near or in the seawater may be directly attacked by the seawater thus exhibiting even higher accelerated corrosion. Very special and expensive alloys such as nickel-chromium and copper-nickel alloys have been developed which exhibit improved corrosion resistent properties in marine or saline environments. However, due to the costs associated with such special alloys, these alloys are not used for roofing materials. Furthermore, when using these various metal materials for architectural purposes, it is generally necessary to provide a dull weathered surface. Such surface coloring was normally caused by exposure to atmosphere; however, with the sulfur content in various locations differing, uniform color was not always guaranteed. When using stainless steel, pre-coloring has been attempted by electrolytic oxidation, by oxidation to change light refraction or by colored coatings. These processes are expensive and not very successful. Using many of these coloring processes, fingerprints often can discolor the surface.
Due to the lack of cost effective building materials that provide excellent corrosion resistance in a marine or saline environment and are properly colored, there has been a demand, especially from consumers located along or near costal regions, for building materials which are not cost prohibitive colored, and provide excellent corrosion resistance especially in marine or saline environments.